Energy potential, flow characteristics and stability of water and alcohol-based rice-straw biochar slurry fuel

Biochar/char slurry (CS) has the potential to be a biofuel in transport and energy applications. This work investigates the effects of varying the liquid suspension medium on the stability, rheology, and higher heating value (HHV) of a biochar-based-slurry biofuel. The slurry was formed with 40 %wt. of rice straw biochar (RSB) and water and ethanol-water mixtures as the suspension mediums. Three surfactants, sodium lignosulfonate (SL), sodium tripolyphosphate (STPP), and sodium dodecylbenzene-sulfonate (SDS), were used in water, whereas only SDS was used within the ethanol-water mixtures. All CSs exhibited a shear thinning behaviour. The lowest viscosities were 548.34, 321.27, and 218.86 mPa s at a shear rate of 100 s−1 for CS prepared with STPP and water, ethanol with SDS, and ethanol, respectively. The highest HHV of 27.75 MJ/kg was found for the CS prepared with ethanol. The stability of ethanol-based CS was lower than water-based CS. An energy balance based on the product yield, HHV and the energy used in pyrolysis shows that, an increase in energy potential of 22.67 MJ/kg than that required in pyrolysis (11.42 MJ/kg). Moreover, the RSB and CS containing ethanol have an energy densification ratio of 1.56 and 1.28, and an energy gain of 81.45% and 93.7%, respectively, compared to rice straw feedstock. The energy assessment revealed that this fuel technology might be a suitable alternative to the traditional pathway of liquid (bio-oil) and gas fuel production via fast pyrolysis and gasification.